The field of Distributed Temperature Sensing (DTS) utilizes sensing fiber optic wires to measure temperatures in remote locations, such as in a pipe or duct. As an example, DTS systems are used in oil, gas and geothermal well environments to determine the temperature along the length of the well.
DTS leverages Raman scattering theory and technology by measuring inelastic scattering of a photon in the sensing fiber to calculate the temperature along the sensing fiber. Raman scattering technology is inherently an intensity-based measurement. More specifically, the derivation of temperature information relies on the measurement of a particular light intensity of two backscattered signals to calculate an intensity ratio of the two backscattered signals and, based on the intensity ratio, derive the desired temperature information. The measurement of the light intensity of the two backscattered signals is very sensitive to errors that can be induced by loss mechanisms not related to temperature.
For instance, when a sensing fiber is exposed to hydrogen, losses are induced from the diffusion of the hydrogen into the glass core of the sensing fiber, which causes the light signal in the sensing fiber to be absorbed. The light absorption properties of the hydrogen are such that they absorb the two backscattered signals differently and, thus, induce a different, direct temperature offset to the calculated value (i.e., different temperature error). The issue of hydrogen diffusion in the sensing fiber is exacerbated in the context of oil, gas and geothermal wells, where hydrogen is present both naturally and in heightened concentrations due to a galvanic reaction of the hydrocarbon well fluids (e.g., oil, gas, etc.) with metal components of the well and DTS system.
As one means of preventing the absorption of hydrogen into the sensing fiber, prior art approaches have encased the sensing fiber of the DTS system in metal structures and specialized coatings to slow down the hydrogen diffusion. By slowing the rate of diffusion of hydrogen into the sensing fiber, the effects of the hydrogen are not seen in a reasonable performance lifetime of the sensing fiber. However, at elevated operating temperatures (e.g., around or above 175° C.), these diffusion barriers or blockers decline in efficacy and the absorption of hydrogen into the sensing fiber becomes a problem. Elevated temperatures in excess of 175° C. are common in the context of some oil, gas and geothermal wells and, thus, attempts to slow or block the absorption of hydrogen have been frustrated.
As an alternative technique, the prior art has modified the material design of the glass of the sensing fiber optic core to greatly minimize reaction with the hydrogen. New core materials, such as a pure SiO2 sensing fiber, eliminate the permanent reaction species, which somewhat reduces the amount of error. Meanwhile, the new core materials continue to allow the reversible absorption effects of hydrogen, which produces unacceptable amounts of error. In short, the new core materials fail to completely eliminate the problem of hydrogen absorption into the sensing fiber.
As one means of compensating for the seemingly-inevitable absorption of the hydrogen into the sensing fiber, prior art techniques have interrogated the sensing fiber to correct for the absorption of hydrogen therein. For example, techniques are known to use either a partial loop architecture or a full loop architecture to interrogate the loss characteristics of the sensing fiber.
Referring to FIG. 1, according to the partial loop architecture, a single-channel optical DTS interrogator 10 measures the temperature of a sensing fiber 12 at two sensing points ZI (I1), ZII (I1+I2) spaced apart around the sensing fiber. The two points Z1 and Z11 are designed to be co-located along a partial looped portion of the sensing fiber 12. Thus it is presumed that the temperature measured at the two co-located sensing points ZI, ZII should be the same. Based on this presumption, any difference in measurement can be attributed to non-temperature loss or error. For instance, from the interrogation, information is derived regarding absolute loss or error that occurs on an intermediate portion (I2) 14 of the sensing fiber 12 that is between the two co-located sensing points ZI, ZII. This derived information is used to compensate for the error and calculate an approximate temperature along the length of the sensing fiber 12.
The partial loop architecture also operates under the assumption that both legs (i.e., the co-located halves of the intermediate portion 14) of the sensing fiber 12 have the same loss over a section of the sensing fiber 12 that is covered by both legs. However, this is not always the case. In fact, asymmetrical loss is common and can have a significant impact on the calculated temperature.
Referring to FIG. 2, in a full loop architecture, a two-channel interrogator 20 measures the temperature of a full-looped sensing fiber 22 at a single point ZIII (I3, I4), but from both ends (i.e., opposing directions). Based on the measured temperatures, the loss of each wavelength is reconstructed and the absolute loss or error in the sensing fiber 22 is determined. The temperature along the sensing fiber 22 is then calculated based on the determined error.
However, neither the partial loop architecture nor the full loop architecture compensates for the entire problem of loss the sensing fiber 12, 22. For instance, both architectures use calculations that rely on averaging of the loss in the sensing fiber 12, 22. As a result, neither of the architectures accounts for non-uniform distribution of DFA and sources such as modal loss that is dynamic, which results in measurement error along discrete points of the sensing fiber.
The object of the present invention is, therefore, to provide a method for DTS measurement, which, among other desirable attributes, significantly reduces or overcomes the above-mentioned deficiencies of known DTS methods.